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The compound KTi(SO4)(2)center dot H2O was recently reported as a quasi-one-dimensional spin-1/2 compound with competing antiferromagnetic nearest-neighbor exchange J(1) and next-nearest-neighbor exchange J(2) along the chain with a frustration ratio alpha = J(2)/J(1) approximate to 0.29 [G. J. Nilsen, H. M. Ronnow, A. M. Lauchli, F. P. A. Fabbiani, J. Sanchez-Benitez, K. V. Kamenev, and A. Harrison, Chem. Mater. 20, 8 (2008)]. Here, we report a microscopically based magnetic model for this compound derived from density functional theory (DFT) based electronic structure calculations along with respective tight-binding models. Our (LSDA+U-d) calculations confirm the quasi-one-dimensional nature of the system with antiferromagnetic J(1) and J(2), but suggest a significantly larger frustration ratio alpha(DFT) approximate to 0.94-1.4, depending on the choice of U-d and structural parameters. Based on transfer matrix renormalization group (TMRG) calculations we find alpha(TMRG) = 1.5. Due to an intrinsic symmetry of the J(1)-J(2) model, our larger frustration ratio alpha is also consistent with the previous thermodynamic data. To identify the frustration ratio alpha unambiguously, we propose performing high-field magnetization and low-temperature susceptibility measurements.

CAMEA is an indirect geometry spectrometer conceived by a Danish (Copenhagen University, Technical University of Denmark) and Swiss (École Polytechnique Fédérale de Lausanne, Paul Scherrer Insitut) Collaboration for the European Spallation Source (ESS), Lund, Sweden. The CAMEA instrument concept has been selected for construction at the ESS, subject to a feasibility study. This document is the final report for the concept phase of the CAMEA instrument project for the European Spallation Source. The material contained within this report represents updated versions of all of the reports submitted for the instrument proposal to the ESS, from which CAMEA was selected for construction.

Nanoparticle-based magnetic resonance imaging T-2 negative agents are of great interest, and much effort is devoted to increasing cell loading capability while maintaining low cytotoxicity. Herein, two classes of mixed-ligand protected magnetic-responsive, bimetallic gold/iron nano particles (Au/Fe NPs) synthesized by a two-step method are presented. Their structure, surface composition, and magnetic properties are characterized. The two classes of sulfonated Au/Fe NPs, with an average diameter of 4 nm, have an average atomic ratio of Au to Fe equal to 7 or 8, which enables the Au/Fe NPs to be superparamagnetic with a blocking temperature of 56 K and 96 K. Furthermore, preliminary cellular studies reveal that both Au/Fe NPs show very limited toxicity. MRI phantom experiments show that r(2)/r(1) ratio of Au/Fe NPs is as high as 670, leading to a 66% reduction in T-2 relaxation time. These nanoparticles provide great versatility and potential for nanopartide-based diagnostics and therapeutic applications and as imaging contrast agents.

In Switzerland the National Science Foundation recently funded a network on Nano-Skyrmionics [https://skyrmions.epfl.ch] joining scientists from various institutions. Here, we provide a status report on selected forefront aspects of the research field. This includes identifying fundamentally new topological magnetization configurations, the process-es of skyrmion nucleation, manipulation, annihilation and detection with a view to their application as parts of memory devices, and their potential integration in broad-band spin dynamic and logic devices.

We propose the construction of a highly innovative spectrometer – CAMEA – offering Continuous Angular and Multiple Energy Analysis. Combining indirect time-of flight with multiple consecutive analyser arrays, this instrument will provide massive flux on the sample and strongly enhanced efficiency in detecting neutrons scattered in the horizontal scattering. The combination yields a spectrometer with completely unprecedented performance - with gains from 2 up to 4 orders of magnitude compared to current state of the art.

Physical review research 5(1), L012010 (2023). doi:10.1103/PhysRevResearch.5.L012010, We examine the role of charge, structural, and spin degrees of freedom in the previously poorly understood phase transition in the 5d1 transition-metal double perovskite Cs$_2$TaCl$_6$ using a combination of computational and experimental techniques. Our heat capacity measurements of single-crystalline Cs$_2$TaCl$_6$ reveal a clear anomaly at the transition temperature, TQ, which was not previously observed in polycrystalline samples. Density functional calculations indicate the emergence of local charge quadrupoles in the cubic phase, mediated by the paramagnetic spins or local structural distortions which then develop into long-range-ordered charge quadrupoles in the tetragonal phase. Our resonant elastic x-ray scattering on Cs$_2$TaCl$_6$ single crystals lends support to our calculations. Our work provides insight into the phase transition in Cs$_2$TaCl$_6$ at TQ, and demonstrates the utility of this unique combination of techniques in understanding the complex physics of hidden orders in paramagnetic spin-orbit-entangled compounds., Published by APS, College Park, MD

This thesis is dedicated to the memory of my late professor A.R. Mackintosh — meant as a student’s tribute to his work of great insight. Through our unfortunately short acquaintance I owe Allan my interest in the field of solid state physics in general and rare earth magnetism in particular. I am grateful to my supervisors Jens Jensen from the Ørsted Laboratory, Niels Bohr Institute and Des McMorrow from the Solid state department, Risø National Laboratory. For their help and suggestions on various problems, I also wish to thank the people in the solid state department, my fellow students and my family. The work falls into three main parts, namely experiments, theory and calculations. On the other hand the examined system shows several different interesting features. I have chosen to start by supplying the background knowledge, occasionally mentioning the implications on the features. Afterwards I discuss each of the features individually. This procedure has led to some overlapping in the report, but on the other hand I found it the most instructive way.

We report the observation by small-angle neutron scattering (SANS) of a magnetic helical structure confined in a thin film of the chiral lattice magnet FeGe. Twofold magnetic Bragg spots appearing below the magnetic transition temperature indicate the formation of a spin helix with a single propagation vector q aligned perpendicular to the film plane. Due to magnetic anisotropy, the direction of q is unaffected by an external magnetic field H. Instead we observe anisotropic deformations of the spin helix with respect to the H direction. In the configuration with H perpendicular to q, the helical pitch exhibits hysteretic elongation with H, while the system tends to maintain an integer number of spiral turns within the film thickness by continuously pushing out one turn. For H parallel to q, the helix is smoothly distorted to a conical structure with minimal change in the magnetic period. The direct measurement of q by SANS establishes a correspondence between helix deformation and macroscopic features observed in magnetization and magnetoresistivity.

We demonstrate quantum critical scaling for an S = 1/2 Heisenberg antiferromagnetic chain compound Cu(C4H4N2)(NO3)(2) in a magnetic field around saturation, by analyzing previously reported magnetization [Y. Kono et al., Phys. Rev. Lett. 114, 037202 (2015)], thermal expansion [J. Rohrkamp et al., J. Phys.: Conf. Ser. 200, 012169 (2010)], and NMR relaxation data [H. Kuhne et al., Phys. Rev. B 80, 045110 (2009)]. The scaling of magnetization is demonstrated through collapsing the data for a range of both temperature and field onto a single curve without making any assumption for a theoretical form. The data collapse is subsequently shown to closely follow the theoretically predicted scaling function without any adjustable parameters. Experimental boundaries for the quantum critical region could be drawn from the variable range beyond which the scaled data deviate from the theoretical function. Similarly to the magnetization, quantum critical scaling of the thermal expansion is also demonstrated. Further, the spin dynamics probed via NMR relaxation rate 1/T-1 close to the saturation is shown to follow the theoretically predicted quantum critical behavior as 1/T-1 proportional to T-0.5 persisting up to temperatures as high as k(B)T similar or equal to J, where J is the exchange coupling constant.

The discovery of multiple coexisting magnetic phases in the crystallographically homogeneous compound Ca3Co2O6 has stimulated ongoing research activity. In recent years the main focus has been on the zero-field-state properties, where exceedingly long timescales have been established. In this study we report a detailed investigation of static and dynamic properties of Ca3Co2O6 across the magnetic field induced transition around 3.5 T. This region has so far been practically neglected, while we argue that in some aspects it represents a simpler version of the transition across the B = 0 state. Investigating the frequency dependence of the ac susceptibility, we reveal that on the high-field side (B > 3.5 T) the response corresponds to a relatively narrow distribution of magnetic clusters. The distribution appears very weakly dependent on magnetic field, with an associated energy barrier of around 200 K. Below 3.5 T a second contribution arises, with much smaller characteristic frequencies and a strong temperature and magnetic field dependence. We discuss these findings in the context of intrachain and interchain clustering of magnetic moments.

Magnetic properties of AMoOPO4Cl (A=K,Rb) with Mo5+ ions in the 4d1 electronic configuration are investigated by magnetization, heat capacity, and nuclear magnetic resonance (NMR) measurements on single crystals, combined with powder neutron diffraction experiments. The magnetization measurements reveal that they are good model compounds for the spin-1/2 J1−J2 square-lattice magnet with the first and second nearest-neighbor interactions. Magnetic transitions are observed at around 6 and 8 K in the K and Rb compounds, respectively. In contrast to the normal Néel-type antiferromagnetic order, the NMR and neutron diffraction experiments find a columnar antiferromagnetic order for each compound, which is stabilized by a dominant antiferromagnetic J2. Both compounds realize the unusual case of two interpenetrating J2 square lattices weakly coupled to each other by J1.

Skyrmions, topologically protected nanometric spin vortices, are being investigated extensively in various magnets(1-11). Among them, many structurally chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature T-c, while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material(12), beta-Mn-type Co8Zn8Mn4, a field-cooling via the equilibrium SkX state can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature and magnetic field. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications.

Skyrmions are nanometric spin whirls that can be stabilized in magnets lacking inversion symmetry. The properties of isolated skyrmions embedded in a ferromagnetic background have been intensively studied. We show that single skyrmions and clusters of skyrmions can also form in the helical phase and investigate theoretically their energetics and dynamics. The helical background provides natural one-dimensional channels along which a skyrmion can move rapidly. In contrast to skyrmions in ferromagnets, the skymion-skyrmion interaction has a strong attractive component and thus skyrmions tend to form clusters with characteristic shapes. These clusters are directly observed in transmission electron microscopy measurements in thin films of Cu$_2$OSeO$_3$. Topological quantization, high mobility and the confinement of skyrmions in channels provided by the helical background may be useful for future spintronics devices., Comment: 5 pages, 3 figures, 4 pages supplement

The first part of this thesis presents the theoretical study of an anomaly of unknown origin in the excitation spectrum of the Quantumspin-1/2 Heisenberg Square lattice Anti-Ferromagnet. The anomaly manifests itself in Inelastic Neutron Scattering data for short wavelength/high energy excitations. Instead of the expected sharp semi-classical harmonic modes, a broad continuum emerges suggesting the possibility of fractionalized excitations. A theoretical framework based on the Gutzwiller projection is developed and allows to link the observed continuum to unbound fractional quasiparticle pairs while the sharp harmonic excitations may be described by bound ones. The second part of this thesis presents the detailed theoretical modeling of the spin-wave dispersion relation measured in insulating cuprate materials. Starting from the one-band Hubbard model with extended hopping amplitudes, an effective low-energy theory is derived allowing to describe on the same footing different insulating cuprate magnetic excitation spectra. The effective theory is fitted against experimental data and microscopic model parameters are extracted. The high level of details included in our effective theory allows a consistent characterization of the studied materials as measured by various magnetic or electronic experimental techniques.

The collective behavior of systems consisting of interacting dipoles is a subject of considerable studies. The anisotropic nature of such interactions opens an arena to explore fundamental questions in correlated electron physics, ranging from quantum entanglement, phase transitions, spin glass states to disorder and fluctuations. LiHoF4 is a textbook example of a ferromagnetic Ising-dipolar model, offering a simple and well-understood Hamiltonian. The system undergoes a quantum phase transition (QPT) in a field transverse to the easy axis, which induces quantum fluctuations between the ground state doublet. Dilution of Ho sites with non-magnetic Yttrium ions lowers only the transition temperature (Tc), and eventually lead to spin-glass state. While Tc decreases in a linear fashion, as expected from simple mean-field (MF) calculation, critical field decreases much faster. The behavior upon dilution has been pointed out to be related to randomness and off-diagonal dipolar interactions. In chapter 5 of this thesis I quantify the deviation of experimental results from neutron scattering studies from MF prediction, with the aim that this analysis can be used in future theoretical efforts towards a quantitative description. The aim of this thesis, however, deals with LiErF4 which is an unexplored planar dipolar antiferromagnetic member of LiReF4 family, with TN ≃ 370 mK. The system undergoes a QPT in an applied field H∥c = 4.0±0.1 kOe, confirmed by a softening of the characteristic excitations at Hc. A combined neutron scattering, specific heat, and magnetic susceptibility study reveals a novel non-MF critical scaling of the classical phase transition, belonging to the 2DXY /h4 universality class. In accord with this, the quantum phase transition at Hc exhibits a three-dimensional classical behavior. The effective dimensional reduction may be a consequence of the intrinsic anisotropic nature of the dipolar interaction. Four-fold anisotropy and degeneracy breaking could be due to the "order-by-disorder" phenomena, which could open a gap in dispersion of the magnetic excitations.

The spin-1/2 Heisenberg antiferromagnets in one and two dimensions are important models in quantum magnetism in which many-body effects play a crucial role. Their collective excitations are associated with quasiparticles the interaction of which are difficult to describe. Neutron scattering offers a very powerful experimental technique to study these excitations. In addition, it can reveal the nature of exotic ground-states favored by competing frustrated interactions or by an applied magnetic field. This thesis presents neutron scattering studies of four materials, each representing a model magnetic system. • Cu(DCOO)2·4D2O (CFTD) is a model realization of the S =1/2 square-lattice antiferromagnet. It displays Néel order and its long-wavelength excitations are well described by renormalized spin-wave theory. In contrast, the excitations at the magnetic zone-boundary display anomalous dispersion and intensity close to (π, 0). A polarized neutron study of the anomaly additionally reveals the presence of a transverse continuum associated with a lineshape that extends to high energies. Results for the transverse and longitudinal continua are compared with Quantum Monte Carlo and fermionic Resonating Valence Bond theories. • (5CAP)2CuCl4 (CAPCC) is a S = 1/2 square-lattice antiferromagnet with modest saturation magnetic field Hs = 3.6 T and ∼ 20% antiferromagnetic interlayer coupling. Its spin excitations, measured above saturation are used to determine the microscopic exchange parameters of its Hamiltonian, while the presence of spontaneous magnon decay is investigated above the theoretically predicted threshold field H*= 0.76Hs. • CuSO4·5D2O is a model realization of the S = 1/2 antiferromagnetic Heisenberg chain. Its excitations are fully characterized in various regimes of applied magnetic field. In zero field, the presence of two- and four-spinon states is quantitatively confirmed, while the development of long-range Néel order is shown to preserve this picture qualitatively. In applied magnetic field, the excitations are studied with polarized neutrons and the existence of exotic psinons, anti-psinons and string-solutions continua is discussed. • LiCuVO4 is a S = 1/2 frustrated multiferroic chain material with competing ferromagnetic nearest and antiferromagnetic next-nearest neighbor interactions. Its magnetic structure is found to be a perfect circular cycloid the chirality of which can be tuned with an applied electric field. In crossed electric and magnetic fields, the behavior of the cycloid closely matches with that theoretically predicted from the spin supercurrent scenario. A magnetic-field induced transition to short-range longitudinal order is also evidenced with polarized neutrons. The present work was carried out in the Triple-Axis Spectrometers group at the Institut Laue Langevin in Grenoble, France and in the Laboratory for Quantum Mangetism at the EPFL, Switzerland.

This TPIV experiment deals with first measurements of CeAlSi and CeAlGe magnetic properties and allows to a comparison between these last two. The first step is a description of MPMS measurement, then the paramagnetism and ferromagnetism theory leads to an essential knowledge for the future analysis of principal magnetic phenomenon. Turning now to practical aspects, this experiment investigates the CeAlSi and CeAlGe magnetization depending on the temperature and on the mag- netic field, which allows to the determination of the Curie-Weiss temperature, the transition temperature and the effective magnetic moment of this two alloys. The study of magnetization as function of the magnetic field shows hysteresis and paramagnetic regimes for both compounds; Brillouin fit has been taken in account. Analytical methods, used to calculate previous above parameters, are given in this report.

TP4 project: My work is mainly to achieve the identification, segmentation and tracking of defects and analyze their dynamics. In detail, the positions, the motion tracks, life expectancy, velocity distribution, etc. We will determine these observables among 488 frames while experimentally the field is ramped continuously from about 600 Oe to 1200 Oe.

A method for decreasing the minimum temperature achievable by a SQUID magnetometer is presented, based on adiabatic demagnetisation refrigeration. The method is found to successfully provide cooling to below 0.26 K for more than an hour. However, performing magnetisation measurements strongly influences the temperature and shortens the duration to a few minutes in the original setup. The cause is most probably re-evaporation of condensed helium, and possible improvements to prevent this effect are suggested.

In fundamental interactions research, dipolar magnets are particularly interesting to study since they provide a large spectrum of quantum phenomena [1-2]. Magnetic dipolar interaction are omnipresent in magnetic systems. However, in most of materials, various exchange interactions within the crystal environment hide the purely dipolar interaction. It has been shown [2-3] that in LiRF4 family, where R is a rare earth element the exchange interactions with nearest neighbours can be neglected. Moreover, by simply changing the R ion in the tetrafluoride lithium matrix, the magnetic behaviour can completely differ at low temperature. For instance LiHoF4 is a Ising ferromagnet [4], while LiErF4 is a XY antiferromagnet [5] and LiTmF4 a Van Vleck paramagnet [6]. In order to understand theses systems, a detailed comprehension of the electronic crystal field is necessary, since the ground state is determined by this interaction [7]. The energy range corresponds typically to the infrared region. While a lot of studies have been done in other LiRF4, LiTmF4 has not be fully investigated. This work presents the spectroscopy analysis from the far to the near infrared regions (10−100000 cm −1) of a pure LiTmF4 single crystal at cryogenic temperatures using Fourier transform Spectroscopy technique. The first part details the structure and the energy scale. The experimental details and FTIR spectroscopy will be then presented. The third chapter is dedicated to explain how the energy levels have been assigned and their strength. The results for the first three manifolds are presented and exhibit some intriguing results such as splitting. Finally, one will discuss the perspectives.

The aim of this project is to compute the spin wave dispersion of the Sr2Cu3O4Cl2 like lattice. After having introduced the necessary theoretical elements and the crystallographic structure of Sr2Cu3O4Cl2, we will first calculate the couplings between two sites of the lattice. The second main part is to compute the spin wave dispersion relation, in function of the coupling previously calculated.

During this project we studied cavities as an alternative method to stripline resonators, to create an oscillating magnetic field. We were able to simulate different cavity designs and reproduce theoretical results as a numerical simulation and experimentally. Thus getting familiar with cavities and studying how their design influences the TE011 mode in which we lay our interest. It was shown that with cavities one could reach a higher quality factor than with stripline resonators. We also found out that in order to make a cavity small enough to be useful it is indispensable to use dielectric materials and studied the behavior of a few possible cavity designs.

As a conclusion, one can say that the main goal of this report was reached: spin wave dispersion was calculated for Nd2Fe14B crystal samples which were also prepared for neutron scattering experiment. Indeed, several steps were made in order to perform such experiment. First, one needs to select a couple of samples and analyse them with X-ray (Laue method) to know the orientation of their reciprocal axis. Then, after checking their quality and their homogeneity, one can place them on a holder and start the neutron experiment. This was successful in our case where 5 samples out of 6 were judged acceptable for the experiment. Computing spin wave dispersion, we found out that the main interactions in Nd2Fe14B were between iron atoms and especially in the (a,b)-plane. However, this study of spin wave was restricted to a phase where all spins were pointing along the c-axis; nothing was done for the canted phase below TS = 135:94 K which could have been interesting but more difficult to obtain.

This TP IV follows the TP IV "Tracking of Skyrmion Motion with matlab" and is using new movie as material. This report presents a description of a code, including an identification and a tracking code, based on new motion hypotheses resulting from the previous study of Skyrmion motion, the main results and observations, finally a statistical analysis of the Skyrmion motion. This study is using a single movie showing current driven skyrmion motion.